The present invention relates to improved methods and apparatus for communicating information over greater distances than otherwise possible on bidirectional media subject to extraneous power, which tends to interfere with the reception of the desired signals ("interference"). Interference includes unwanted disturbances superimposed upon a useful signal, which tend to obscure its information content, and undesired disturbances within the useful frequency band, e.g., (1) disturbances produced by other information communication services and (2) undesired energy appearing in one signal path of a bidirectional medium as a result of coupling thereto from one or more other signal paths in the medium ("crosstalk").
The signal, i.e., the intelligence, message, or effect, to be conveyed over a communication system, is coupled to a bidirectional medium by a directional coupler, i.e., a device at a transceiver used to separate energy to be transmitted from energy to be received, e.g., a hybrid circuit, an analog echo-canceler, a digital echo-canceler, a frequency splitter with echo-cancellation, or a 2wire-4wire ("2W-4W") convertor. The bidirectional media can be radio or microwave links through air or outer space, or copper wire twisted pairs.
In telephony, copper wire twisted pairs are typically placed in bundles or groups of 50 pairs per cable; see S. V. Ahamed, P. P. Bohn and N. L. Gottfried, "A Tutorial on Two-Wire Digital Transmission in the Loop Plant," IEEE Transactions on Communications, vol. COM-29, no. 11, p. 1554 (November 1981); J. Werner, "The HDSL Environment," IEEE Journal In Selected Areas In Communications, vol. 9, No. 6, p. 785 (Aug. 1991). Twisted wire pairs carry signals between a central office and a plurality of subscribers ("local loop"). Signals conveyed on copper wire twisted pairs are susceptible not only to crosstalk, but also to other interference such as thermal noise, DC wander, jitter, intersymbol interference ("ISI"), and quantization noise.
Crosstalk is generally classifiable into two kinds, based on the location of the interfering transmitter with reference to the bidirectional media into which the crosstalk is coupled, i.e., near-end crosstalk ("NEXT") and far-end crosstalk ("FEXT"). Interfering signal transmitters located at the same end of the bidirectional media as the signal receiver of interest cause near-end crosstalk, whereas interfering signal transmitters located at the end of the bidirectional media remote from the signal receiver of interest cause far-end crosstalk. Crosstalk is sub-classifiable into self-crosstalk, i.e., interference coupled from transmitters to receivers of the same kind, and inter-crosstalk, i.e., interference coupled from transmitters to receivers of a different kind. From the viewpoint of the receiver of interest, inter-crosstalk includes crosstalk coupled into the bidirectional medium to which the transmitter of interest is connected ("ingress crosstalk") and crosstalk coupled out of the bidirectional medium to which the receiver of interest is connected ("egress crosstalk").
Crosstalk becomes an increasingly troublesome undesirable signal impairment as a function of increasing frequency. Crosstalk on copper wire twisted pairs can be the limiting impairment for symmetric transmission on the local loop. Because near-end crosstalk from interfering transmitters at the central office or at a remote location is not attenuated by transmission loss over the length of the bidirectional medium, near-end crosstalk is potentially the dominant impairment of signal communication on the medium. Far-end crosstalk is only significant at those frequencies where near-end crosstalk is not present or is small. Far-end crosstalk is typically less severe an impairment than near-end crosstalk, because far-end crosstalk is attenuated by traveling over the full length of the bidirectional medium.
Conventional services on a digital subscriber loop, such as "BR-ISDN" or "BRI" (basic-rate integrated services digital network ("ISDN") service), and conventional high-bit-rate digital subscriber loop service ("HDSL"), are communications systems designed to employ symmetric echo-canceled transmission. As a result, self-NEXT is the dominant crosstalk impairment in such systems, limiting the range of communication on the loop. These conventional symmetric digital subscriber loop ("SDSL") communication systems are designed to tolerate the 1% worst case crosstalk interference from a full binder group of services of the same kind. Interference due to crosstalk into either BRI or HDSL services from different services is less significant an impairment to BRI or HDSL than self NEXT.
The conventional asymmetric digital subscriber loop ("ADSL") communication system, on the other hand, originated as a frequency division multiplexed ("FDM") communication system, where the transmitted signals over the bidirectional medium do not share the same spectrum, in whole or in part. By employing bidirectional transmission in non-overlapped bandwidths, the conventional FDM system eliminated self near-end crosstalk as a range limiting impairment and eliminated any requirement for echo-cancellation. The range of the conventional ADSL service is limited by near-end crosstalk from different services, e.g., the conventional BRI and the HDSL systems, and far-end crosstalk from other conventional ADSL systems. The conventional ADSL system employs asymmetric data transmission rates, which makes the ADSL service spectrally incompatible with T1 carrier, both from an ingress and also an egress point of view, when the T1 carrier is in the same or in an adjacent binder group with respect to the ADSL system. It is recognized that ANSI T1.413 includes an echo-cancellation option for ADSL service, but because of the asymmetry of the transmission rates, ADSL is still largely an FDM design.
Another conventional technique for designing symmetric digital subscriber loop systems is time division multiplexing or time compression multiplexing of bidirectional transmissions ("ping-pong transmission"). Ping-pong transmission has been proposed for the very high-speed digital subscriber line ("VDSL") systems under the name synchronized DMT ("SDMF"). As is the case for the FDM system, ping-pong transmission eliminates self near-end crosstalk as a signal range limiting impairment; inter-crosstalk is the impairment which limits ping-pong transmission services.
Conventional FDM systems and symmetric echo-canceled transmission systems have been proposed for the prospective single-pair HDSL ("HDSL2") system. The prospective HDSL2 system has the following specifications: (1) symmetric data transmission at a 1.552 Mbps data transmission rate, which includes 1.544 Mbps for information and 8 kbps for framing and overhead used for maintenance and performance monitoring; (2) a 5-6 dB margin in the presence of worst case crosstalk; and (3) inter-crosstalk from the HDSL2 system must not be the limiting impairment for existing services.
None of the conventional transmission techniques proposed for implementing the HDSL2 service in a telephone company carrier service area ("CSA") range is able to satisfy these HDSL2 system specifications; cf. TR-28, "A Technical Report on High-Bit-Rate Digital Subscriber Lines (HDSL)," prepared by the T1E1.4 Working Group in Digital Subscriber Lines (February 1994) with respect to CSA considerations. Although the conventional FDM system is thought to be capable of reaching the CSA range, the FDM technique has proven inadequate because of unacceptable levels of ingress and egress inter-crosstalk, especially with respect to T1 carrier service.
The failure of conventional techniques to meet the CSA range requirement for transmission of HDSL2 service on a single copper wire twisted pair at the specified symmetric transmission rate of 1.552 Mbps is caused by the fact that each of these conventional techniques operates at an extreme. For the conventional symmetric, echo-canceled system, the extreme is the dominant impairment of self-NEXT. For the conventional FDM and the conventional ping-pong transmission techniques, the extreme is the dominant range-limiting impairment of inter-crosstalk with respect to different services in the same or adjacent binder group, even though self-NEXT is completely eliminated.
The information communication system art requires a new transmission technique for communicating information over greater distances than are otherwise possible on bidirectional media subject to interference, which technique does not possess the drawbacks of conventional techniques.
It is, therefore, a principal object of the present invention to increase the range of signal communications on bidirectional media by reducing the effects of interference.
It is another object of the present invention to provide a new signal transmission technique that reduces self-NEXT while minimizing interference both with existing services and also prospective services.
It is a further object of the present invention to increase the range of a high-speed data transmission system for the local loop, by reducing self-near end crosstalk to the level of inter-crosstalk from other services.
It is yet another object of the present invention to minimize egress inter-crosstalk interference into existing services, while at the same time increasing the range of signal communication on bidirectional media.